Coordinate measuring apparatus having a probe in the form of a solid-state oscillator

ABSTRACT

The solid-state oscillator, preferably a watch crystal (1/2), is mounted with its direction (S) of vibration at an angle to all three measuring directions (x, y, z) of the coordinate measuring apparatus. The oscillator circuit of the solid-state oscillator is connected to an electronic circuit which generates two different contact signals. Both measures make it possible to reduce the dependency of the contact uncertainty of the measuring direction.

BACKGROUND OF THE INVENTION

In recent times, probes for coordinate measuring apparatus have becomeknown which apply only a very slight measuring force of less than 1 mNto the workpiece to be measured. These probes operate with a so-calledsolid-state oscillator having an amplitude of vibration which changeswhen approaching the object to be measured. Such a probe is disclosed inGerman utility model 9,213,059.

This known probe has a thin glass rod as a probe element which isattached with adhesive to a prong of a tuning fork crystal in such amanner that the longitudinal axis of the glass rod is aligned parallelto the direction of vibration of the tuning fork crystal.

When a probe of this kind is used in a coordinate measuring apparatusand when the longitudinal axis of the rod or probe element is alignedparallel to one of the measuring directions of the coordinate measuringapparatus as shown in FIG. 1, it can then be determined that theattainable contact uncertainties in the coordinate direction z (that is,in the longitudinal direction of the rod 3a and parallel to thedirection S of vibration of the tuning fork crystal) are significantlyless than the contact uncertainties which are measured in the coordinatedirections x and y perpendicular to the longitudinal axis of the rod 3a.The measuring uncertainties lie apart from each other by a factor of 5.While, for example, a contact uncertainty of only 1.0 μm was determinedfor the measuring axis z, this uncertainty amounts to about 5 μm for thecontact directions x and y.

An asymmetry of this kind in the contact uncertainty is, however,unwanted because many sides must be contacted when making measurementsof geometric elements such as bores, et cetera. The measurementuncertainty should correspond for all contact points to the same valuespecified for the measuring apparatus.

A further problem when working with the above-mentioned probe isobtaining signals which announce clearly and reliably the contact oftime probe element with the workpiece to be measured to the control ofthe coordinate measuring apparatus. If this time point is not clearlyand reproducibly determined, then this likewise increases the contactuncertainty of the probe. The circuit described in German utility model9,213,059 does supply usable results. However, the circuit is rathersensitive to external disturbances. For example, the transfer ofmeasured values is influenced by turbulences in the air. When suchdisturbances occur shortly ahead of contact on the workpiece surface,then these disturbances are not clearly recognized as disturbances and afalse coordinate measuring value is supplied, that is, thesedisturbances contribute to an increased contact uncertainty.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the invention, in a coordinatemeasuring apparatus having a probe of the kind referred to above, to somount the probe or provide a circuit therefor that equal contactuncertainties reliably result, which are as low as possible, in thethree measuring directions x, y and z.

In the arrangement of FIG. 2, the solid-state oscillator (1/2) ismounted with respect to the direction S of vibration at an angle ofpreferably 45° to all three measurement directions x, y and z of thecoordinate measuring apparatus. With the arrangement shown in FIG. 2,contact uncertainties of 2.5 μm result in all three contact directionswith these contact uncertainties being low and comparable. However, thesketched arrangement is disadvantageous insofar as the probe element,that is the rod 3a, is inclined to the three contact directions whichare possible in the coordinate measuring apparatus. The objects O to bemeasured are, as a rule, clamped to the table of the coordinatemeasuring apparatus so as to be aligned to the measuring axes x, y andz. For this reason, measurement is improved when the probe element, thatis, the rod placed on the solid-state oscillator, is aligned parallel toone of the three measuring directions of the coordinate measuringapparatus. For this reason, it is purposeful when at least the contactend of the rod is parallel to one of the measuring directions of thecoordinate measuring apparatus and therefore aligned at an angle to thedirection S of vibration of the solid-state body, for example, in thatthe rod is correspondingly angled or is attached at an angle to thesolid-state oscillator without a bend.

The reliability and contact uncertainty of the above-mentioned probe canbe improved utilizing an electronic circuit which generates at least twodifferent signals characterizing the contact of the probe element withthe workpiece to be measured. With the aid of the two signals occurringcoincidentally in the case of contact, disturbances which could lead toerroneous contacts are effectively detected and suppressed. At thispoint, reference is expressly made to United States patent applicationSer. No. 08/304,709, filed Sep. 12, 1994, now U.S. Pat. No. 5,526,576,which is incorporated herein by reference. In this application, eventhough for another probe type, detailed embodiments for theconfiguration of a reliable and functionally safe electronic circuit forprocessing a probe signal are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 is a simplified concept schematic which shows the case of thealignment of the direction S of vibration of the solid-state oscillatorparallel to a measurement direction z;

FIG. 2 is a simplified concept schematic which shows the case of thealignment of the solid-state oscillator at an angle to all threemeasurement axes in accordance with a first embodiment of the invention;

FIG. 3 is a preferred embodiment of the invention wherein thesolid-state oscillator is mounted at an angle in space and a probeelement arranged at an angle parallel to the measurement axis z;

FIG. 4 shows a further embodiment of the invention having a solid-stateoscillator mounted at a spatial angle and a probe element alignedparallel to the measurement axis z;

FIG. 5 is a concept schematic which shows the most essential componentsof the evaluation electronics connected to the solid-state oscillator;

FIG. 6 is a block circuit diagram of the electronic portion 30 of FIG. 5connected to the solid-state oscillator; and,

FIG. 7 is a schematic which shows the time-dependent trace of thevibration amplitude of the solid-state oscillator during a contactoperation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, a so-called vibrating quartz probe is shown as described in asimilar manner in German utility model 9,213,059. A tuning fork crystalserves as a solid-state oscillator and is, for example, also used inquartz watches. The actual fork-shaped quartz is seated in a frame 1with the electrical terminals and thin glass rods 3a and 3b are glued tothe forward ends of the prongs, respectively, of the fork. Whereas theone glass rod 3a defines the actual probe element with the ball meltedonto the forward end thereof, the second rod 3b is provided to make theoscillator symmetrical. The second rod 3b is glued to the other prong 2bof the crystal and has the same size and same weight.

FIG. 1 shows the conventional alignment of the solid-state oscillatorwith its direction S of vibration parallel to the measurement axis z ofthe coordinate measuring apparatus. In this alignment in the coordinatemeasuring apparatus, in which the solid-state oscillator is utilized,the differences of the contact uncertainty discussed initially resultreferred to the measuring direction.

These asymmetries are avoided according to the invention with thearrangement of the solid-state oscillator shown at an angle in space inFIG. 2. The tuning fork crystal is built unto the measuring arm (notshown) of the coordinate measuring apparatus so that the direction S ofvibration of the two fork prongs (2a and 2b) is aligned at an angle of45° to all three measuring axes x, y and z of the coordinate measuringapparatus. Accordingly, the same low contact uncertainties of 2.5 μm forall three measuring directions result when contacting in the threeabove-mentioned directions.

In the preferred embodiment of FIG. 3, it is shown in detail how theprobe is structurally configured. The probe has a cylindrical housing 10in which the circuit board 14 is placed. The circuit board 14 has theelectronic circuit for exciting the oscillator crystal and forprocessing the probe signal. The tuning fork crystal with its mount 1 isattached at an angle of 45° to the axis of symmetry of the housing atthe lower end of the circuit board 14.

Two glass rods are attached to the two ends of the prongs (2a and 2b) ofthe tuning fork crystal. The larger end of the glass rods, which facesaway from the tuning fork crystal, is offset parallel to the housingaxis.

The lower end 11 of the housing 10 is conically configured and has acentral opening 12. The contact end of the glass rod 13a passes throughthis central opening 12. This conical formation of the lower housing endis facilitated by the spatial angle arrangement of the tuning forkcrystal and improves the accessibility when measuring with the probe intight workpieces. The length of the glass rod 13a is limited toapproximately 10 mm when using conventional watch crystals because ofthe maximum permissible mass for vibrations on the prongs of the tuningfork.

A plastic sleeve 15 is placed around the housing 10 and is likewiseconically formed at its forward end. The plastic sleeve 15 is pushedinto the position shown in phantom outline to protect the probe when notin use. Two annular slots (16a and 16b) in the housing 10 secure thehousing in the two preferred positions in combination with an annularbead at the upper end of the plastic sleeve 15.

The housing 10 is so rotated when mounted in the coordinate measuringapparatus that the tuning fork (2a/2b) of the crystal and therefore thedirection S of vibration lies in one plane as shown. This plane isaligned at an angle of preferably 45° to the two horizontal coordinatedirections x and y.

With the probe described, the surfaces, which are usually aligned in thez-direction, and bores of workpieces are easily contacted which relatesto their accessibility. At the same time, the same values for contactuncertainty result in all three coordinate directions.

This effect can be achieved also with the somewhat modified embodimentshown in FIG. 4. Here, the two glass rods (23a and 23b) are welded at anangle to the prongs (2a and 2b) of the tuning fork crystal and arewithout an offset. Welding is performed either with the aid of a laserbeam or an electric arc.

In both embodiments, a solid-state oscillator is shown having a singledirection of vibration which is aligned at an angle to the measuringdirections of the coordinate measuring apparatus. In this way,comparably large components of vibration of the probe element result inthe three measuring directions.

It is, however, also possible to drive the contact element with asolid-state oscillator which vibrates simultaneously in several spatialdirections or excites several vibration modes of the probe element.Comparably small contact uncertainties in all three spatial directionscan then be realized when it is ensured that comparably large vibrationamplitudes of the probe element result in the three spatial directions.

As shown in FIG. 5, the solid-state oscillator with the connecting leadson its mount 1 is connected via lines (a) and (b) to an electroniccircuit 30. As shown in connection with FIG. 6, this electronic circuit30 includes an oscillator circuit for maintaining the vibration of theprobe element as well as comparators for generating two differentsignals A1 and A2 when the probe element contacts the workpiece to bemeasured. The two signals A1 and A2 are both supplied to the controlunit 31 of the coordinate measuring apparatus which, in turn,communicates via a data bus with the computer 32 of the coordinatemeasuring apparatus. The connections of the control unit 31 to thedrives of the coordinate measuring system as well as to the lengthmeasuring systems are not shown. The length measuring systems measurethe position of the movable measuring slide of the coordinate measuringapparatus in the three spatial directions x, y and z.

The oscillator circuit is identified in FIG. 6 by 33. The solid-stateoscillator, that is, the tuning fork crystal, is driven at a constantfrequency via the oscillator circuit. Furthermore, a control circuit isprovided via which the amplitude of the oscillation is held constant.This circuit comprises a controller 36 to which the output signal of thedigital-to-analog converter 35 is supplied as a reference variable. Thisdesired value can be inputted either manually on the electronic circuitboard 30 via a suitable manually settable microswitch array or with theaid of a microprocessor, for example, by the control unit 31 or thecomputer 32. The controller 36 compares the output signal of thedigital-to-analog converter 35 to the actual value of the signal of theoscillator circuit 33, that is, with the amplitude of the vibration ofthe crystal. The signal of the oscillator circuit 33 is rectified by arectifier 34. The corresponding control signal outputted by thecontroller 36 is fed back via a limiter 37 to the oscillator circuit 33.With these measures, and when approaching the surface of the workpieceto be measured, the amplitude of the vibration of the solid-stateoscillator is maintained constant until the limit voltage of the limiter37 is reached. At this time point, and when coming closer to the surfaceof the object, the amplitude of the vibration begins to drop because ofthe ever greater attenuation until the vibration finally stopsaltogether. This time-dependent trace of the vibration during a contactoperation is shown as exemplary in FIG. 7.

To obtain a probe signal, the output of the rectifier 34 is connected toa difference amplifier 38. The other input of the difference amplifieris likewise connected to the digital-to-analog converter 35. The outputof the difference amplifier 38 is supplied to the inputs of twocomparators (41 and 42). The thresholds of the comparators can belikewise adjusted digitally via a microprocessor or via a manuallysettable microswitch array on the circuit board of the circuit 30 ofFIG. 6. Two digital-to-analog converters 39 and 40 convert the adjustedthreshold values to the analog inputs of the comparators (41 and 42).

The threshold value of the comparator 41 is so adjusted that a fall-offof 5% of the vibration amplitude already permits the comparator 41 tobecome conductive and the first contact signal A1 is generated. Withthis signal, the counter positions of the length measuring systems ofthe coordinate measuring apparatus are frozen. This signal defines thecontact time point.

In contrast, the threshold S2 of the comparator 42 is set to a low valueof, for example, 40% of the signal amplitude of the vibration.Correspondingly, the comparator 2 supplies a signal to the line A2 at alater time point when the vibration is attenuated more in the course ofthe contact operation.

The time delay between the two signals is dependent upon the contactvelocity and is checked by the microprocessor in the control unit 31 asto time coincidence within a time window t_(F) which is correlated tothe contact velocity. A contact is declared to be valid only when thereis a simultaneous occurrence within the above-mentioned time window.Then, the counts of the counters, which were already frozen with thefirst occurrence of the signal on the line A1, are adopted as contactcoordinates.

In the described embodiment, the derivation of the signals A1 and A2results from the amplitude of the vibration of the solid-stateoscillator with the aid of comparators. However, it is also possible toevaluate the frequency shift on the basis of the increasing attenuationof the contact element during a contact operation. This can also be donein combination with the evaluation of the vibration amplitude. In thiscase, it is purposeful to provide digitally adjustable frequency filtersin the evaluation electronic circuit. Likewise, it can be purposeful tomake the limit voltage of the limiter 37 adjustable referred to theparticular measuring task because this limit voltage primarily definesthe contact of the probe element with the workpiece to be measured.

Finally, it is also possible to plot the entire time-dependent trace ofthe vibration amplitude and/or frequency during a contact operation inorder to determine the exact contact time point and to compare thetraces to stored signal traces in order to derive the exact contactcoordinates therefrom. The corresponding procedure is described in DE-OS4,204,602 to which reference is expressly made at this point.

We claim:
 1. A coordinate measuring apparatus defining three measuringdirections (x, y, z), the apparatus comprising:a solid-state oscillatorhaving a direction (S) of vibration; a thin rod defining a probe elementand being mounted on said solid-state oscillator; and, said solid-stateoscillator being mounted relative to said three measuring directions soas to cause said direction (S) of vibration to be at an angle to allthree of said measuring directions, so that said direction (S) ofvibration is inclined to all three of said measuring directions.
 2. Thecoordinate measuring apparatus of claim 1, said thin rod having a firstsegment attached to said solid-state oscillator and a second segmentdefining a contacting end of said rod; and, at least said second segmentextending parallel to one of said measuring directions as well as beingaligned at an angle with respect to said direction (S) of vibration. 3.The coordinate measuring apparatus of claim 2, wherein said secondsegment is bent so as to be parallel to said measuring direction (z) andsaid first segment is aligned parallel to the direction (S) of saidvibration.
 4. The coordinate measuring apparatus of claim 1, said thinrod being linear and being attached at an angle to said solid-stateoscillator.
 5. The coordinate measuring apparatus of claim 2, furthercomprising a housing for accommodating said solid-state oscillatortherein; and, said housing having a portion which is conically shaped toterminate with a central opening through which said probe elementextends.
 6. A coordinate measuring apparatus defining three measuringdirections (x, y, z), the apparatus comprising:a solid-state oscillatorhaving a direction (S) of vibration; a thin rod defining a probe elementand being mounted on said solid-state oscillator; and, said solid-stateoscillator be mounted relative to said three measuring directions so asto cause said direction (S) of vibration to be at an angle to all threeof said measuring directions, said direction (S) of vibration being atan angle of 45° with respect to all three of said measuring directions.